This invention relates to a calibration apparatus, and in particular to a calibration apparatus which makes equal the phases at the antenna elements of an array antenna.
Digital cellular wireless communications systems using DS-CDMA technology are being developed as next-generation mobile communication systems. CDMA methods allocate channels based on codes to perform communication simultaneously; due to interference from the signals of other channels through which there is simultaneous communication, the number of channels through which simultaneous communication is possible, that is, the channel capacity, is limited. Techniques to suppress interference are effective for increasing the channel capacity.
An adaptive array antenna adaptively forms a beam for a desired user according to the environment, and forms a null point for a user which is a large interference source, enabling increases in channel capacity. That is, a beam is formed in the direction of a designated user, and a null point is directed toward a user who is a large interference source, so that radio waves can be received with good sensitivity from the designated user, without receiving radio waves from the large interference source. By this means, the amount of interference can be reduced, and consequently the channel capacity can be increased.
An adaptive antenna uses phase differences at the antenna elements to generate a beam. Hence if phase fluctuations in the wireless portions of the antennas each differ, a beam pattern cannot be controlled correctly. Therefore, in order to correctly control the beam pattern, phase differences at antenna elements arising from different phase fluctuations must be corrected. Means for correcting such phase differences multiplexes a calibration signal, and detects phase differences in the multiplexed signal (see for example Japanese Patent Laid-open No. 2003-218621).
FIG. 14 is a diagram of the configuration of a transmitter comprising a conventional calibration function as described in the above-mentioned patent reference, which makes equal the phase characteristic and amplitude characteristic of signals radiated from antenna elements 1-1 to 1-6 comprised by a linear array antenna. To this end, the transmitter comprises a calibration signal generator 4 which generates calibration signals, an adder 5 to add calibration signals to user multiplexed signals (main signals), a circulator 6 which retrieves signals electromagnetically combined from adjacent antenna elements, a receiver 7 which receives the retrieved signals from the circulator 6, an RF switch 8 which switches the input signals of the receiver 7, a calibration coefficient calculation portion 9 which detects calibration signals from the output of the receiver 7 and calculates calibration coefficients, a multiplier 10 which multiplies calibration coefficients calculated by the calibration coefficient calculation portion 9 and the main signal, a power combiner 11 which combines signals electromagnetically coupled from the antenna elements 1-2, 1-5 adjacent to the antenna elements 1-1, 1-6 on the two ends of the linear array antenna, a user signal multiplexing portion 12 which multiplexes a plurality of user signals, and a beam former 13 which forms a transmission beam pattern by independently controlling the phases and amplitudes of signals input to each of the array antenna elements 1-1 to 1-6 such that the peak is directed in the direction in which the user (mobile station) exists; the calibration signals C1 to C6 generated by the calibration signal generator 4 are orthogonal signals with no correlation, and enable independent retrieval of each calibration signal by the receiver 7.
Calibration signals C1, C3 transmitted from the antenna elements 1-1 and 1-3 are received by the antenna element 1-2 through the electromagnetic coupler between the antenna elements. The received signals C1, C3 are retrieved by the circulator 6 and input to the P1 port of the RF switch 8. Similarly, C2 and C4 are input to the P2 port, C3 and C5 are input to the P3 port, C4 and C6 are input to the P4 port, and C2 and C5 are input to the P5 port.
The RF switch 8 is switched in order, and the signals input to the ports P1 to P5 are demodulated by the receiver 7 and converted to baseband signals; the calibration coefficient calculation portion 9 measures the phases of each of the calibration signals C1 to C6, and determines the calibration coefficients needed to make equal the phases of all of the signals C1 to C6. C1 to C6 are input to the transmitter 3 with the same phase, and so the measures phases of the signals C1 to C6 indicate the scattering in the phase characteristics of the transmitter 3, the antenna elements 1, and cables. Hence by using the multiplier portion 10 to multiply the calibration coefficients calculated from the measured values with the input signals, the phase characteristics can be made equal for each of the transmission systems.
In the technology of the prior art, all the antenna elements must electromagnetically receive calibration signals radiated from adjacent antenna elements. Consequently in the prior art, a configuration is employed in which an RF switch is used to switch the antenna elements and input signals to the wireless reception portion. However, if the number of antenna elements is N, then the RF switch must be able to switch (N−1) signals. The greater the number of signals switched, the more expensive is the RF switch, and so there has been the problem that configurations of the prior art require an expensive RF switch.